1. Field of the Invention
The present invention relates to an ignition apparatus which includes an ignition coil and an ignition circuit integrally united by potting resin.
2. Description of Related Art
One type of known ignition coil is insulated by filling potting resin in a coil case having primary windings, secondary windings and a core therein. One type of known ignition circuit for switching primary current of the ignition coil is an encapsulated ignition circuit (e.g., an integrated circuit die chip) molded into an epoxy resin. Furthermore, the integrated ignition coil and ignition circuit have also been utilized to minimize and simplify the ignition apparatus.
One type of known coil/circuit unification method is to bury the encapsulated ignition circuit in the potting resin of the ignition coil. Since the coefficient of linear expansion of the potting resin of the ignition coil (30-60.times.10.sup.-6) is much larger than that of the components of the mold ignition circuit (3.5-25.times.10.sup.-6), large stress is applied to circuit elements of the encapsulated ignition circuit from the potting resin due to temperature changes. Such stress may cause peeling at a bonding portion (solder bonding portion or wire bonding portion) of the circuit elements or a crack, thereby causing failure of the ignition circuit.
In order to solve the above problem, another type of known ignition apparatus has an encapsulated ignition circuit wrapped in a cushioning material, such as soft resin, and buried in the potting resin of the ignition coil. With such an ignition apparatus, peeling at a bonding area of the circuit elements or the crack can be prevented by absorbing the linear expansion difference between the potting resin and the components of the encapsulated ignition circuit with a deformation of the cushioning material, and by minimizing the stress which is applied to the circuit elements of the mold ignition circuit from the potting resin.
Such an ignition apparatus, however, includes additional costs for the cushioning material needed for wrapping the whole mold ignition circuit. Furthermore, such cushioning material prevents the mold ignition circuit from radiating heat, thereby compromising the durability and the credibility of the ignition circuit.